Devices for the movement of gases are widely utilized. The very first aircraft engines were piston driven propellers. They worked by coupling a piston engine to a propeller. Their simplicity lead to widespread adoption until jet engines were invented. Turbojet engines work by the principle of coupling a turbine to a fuel combination system. Spinning of the turbine compresses a fuel-air mixture which, when burned, provides thrust and torque to rotate the turbine. The first turbojet engines derived their thrust from exhaust leaving the engines. Modern variants of the turbojet engines include turbo prop and turbofan engines, which use torque generated by the exhaust to drive a propeller or fan in addition to compressing the fuel-air mixture. Rocket engines are possibly one of the oldest mechanical propulsion systems, and have not changed much since their inception. A rocket comprises a tube or cone in which sits (or into which is fed) a fuel oxidizer mixture. Expanding gas from combustion of this mixture creates thrust. Rockets, while offering the highest fuel-thrust ratio of any existing propulsion systems, cannot easily vary the amount of thrust they generate. Even adding an ability to turn a rocket on or off significantly complicates its design.
The correct explanation for the action of Crookes radiometer derives from work that Osborne Reynolds submitted to the Royal Society in early 1879. He described the flow of gas through porous plates caused by a temperature difference on opposing sides of the plates which he called “thermal transpiration.” Gas at uniform pressure flows through a porous plate from cold to hot. If the plates cannot move, equilibrium is reached when the ratio of pressures on either side is the square root of the ratio of absolute temperatures. Reynolds' paper also discussed Crookes radiometer. Consider the edges of the radiometer vanes. The edge of the warmer side imparts a higher force to obliquely striking gas molecules than the cold edge. This effect causes gas to move across the temperature gradient at the edge surface. The vane moves away from the heated gas and towards the cooler gas, with the gas passing around the edge of the vanes in the opposite direction. Maxwell also referred to Reynolds' paper, which prompted him to write his own paper, “On stresses in rarefied gases arising from inequalities of temperature.” Maxwell's paper, which both credited and criticized Reynolds, was published in the Philosophical Transactions of the Royal Society in late 1879, appearing prior to the publication of Reynolds' paper. See, Philip Gibbs in “The Physics and Relativity FAQ,” 2006, at math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html.
Despite the descriptions by Reynolds and Maxwell of thermally driven gas flow on a surface dating from the late 19th century, the potential for movement of gases by interaction with hot and cold surfaces has not been fully realized. Operation of a Crookes radiometer requires rarefied gas (i.e. a gas whose pressure is much less than atmospheric pressure), and the flow of gas through porous plates does not yield usable thrust, partially due to the thickness and due to the random arrangement of pores in the porous plates.
Filters are commonly used to filter particulate and other material from a fluid stream such as, for example, a gas stream. Filters typically use fibrous materials to remove solid particles, such as, for example, dust, pollen, mold, bacteria, pollutants, etc., from an air or gas stream. The material used for filtration can include, for example, foam, pleated paper, spun fiberglass elements, cotton, etc. Chemical air filters use and absorbent, or catalyst, for the removal of airborne molecular contaminants, such as, for example, volatile organic compounds or ozone. Air ionizers use fibers or elements with a static electric charge, which attracts dust and other particulates.
In addition to air and gas, water and other fluids are also the subject of filtering. A water filter removes impurities from water by means of a fine physical barrier, a chemical process or a biological process. Filters cleanse water to various extents for irrigation, drinking, aquariums and swimming pools. Types of water filters include, but are not limited to, media filters, screen filters, sieve filters, disk filters, slow sand filter beds, rapid sand filters, cloth filters, etc.
Typically, filters must be serviced, or cleaned, on a regular basis to keep them from getting clogged. This is especially true when the filter is designed to filter out a high magnitude of particulates. A clogged filter can significantly restrict the fluid or gas flow, resulting in a breakdown of the filter and possibly the device or equipment to which it is attached. Additionally, the filter must be sized appropriately to handle the volume of fluid or gas it will receive so as to not unduly restrict the fluid/gas flow.